This invention relates to a laser beam condensing optical device used for laser beam machining, and particularly to a method and a device for focusing or condensing a laser beam on a focal point.
Today, with the appearance of lasers capable of generating high-energy laser beams, such as CO.sub.2 lasers, YAG lasers and excimer lasers, it is possible to cut, weld and surface-treat different kinds of materials by laser beam machining. Such laser beam machining is carried out by guiding a laser beam generated by a laser beam generator through a laser beam guide means to a machining head, condensing the laser beam to a high energy density with a beam condensing optical device in the machining head, and irradiating a workpiece with the thus condensed laser beam.
This beam condensing optical device usually includes condensing lenses and off-axis parabolic mirrors as main optical parts. Condensing lenses are condensing means of a type adapted to bend light beams by passing them. They are optical parts made of ZnSe (zinc selenide), which is a compound semiconductor material, and have a plano-convex or meniscus shape.
ZnSe is a material which is particularly low in its infrared-absorbing power. Thus, it is widely used as the material for the lenses in CO.sub.2 lasers which generate infrared laser beams. But it is impossible to completely prevent the absorption of infrared light. Thus, a ZnSe lens tends to heat up by absorbing energy when a laser beam passes therethrough. This may cause thermal deformation of the condensing lens, a change in its refractive index, or a shift in the focal point, resulting in a reduction in the quality of the condensed beam.
The higher the laser output, the more conspicuously these phenomena will appear. If this condensing lens is used in an environment where its surface tends to be stained with dirt, the dirt itself will absorb laser beam energy, accelerating a temperature rise of the lens, so that the above phenomena will appear still more conspicuously. In the worst case, the lens might burn out.
Thus, such a condensing lens has to be cooled by circulating cooling water along its sides or by spraying a cooling gas on the lens surface. But with any cooling method, it is difficult to efficiently cool the central portion of the lens. Thus, such a lens can be used only in comparatively low-output laser beam machining such as for cutting a thin plate.
A parabolic mirror has a reflective surface in the shape of a paraboloid, a kind of axisymmetric aspheric surface. It is a reflective type of condensing means which utilize the principle that a parallel beam incident on a paraboloid is condensed while creating no aberration. Such a parabolic mirror can reflect and focus a laser beam with no aberration, if the incident laser beam has an optical axis strictly parallel to the axis of the parabolic surface and also is a strictly parallel beam with no divergence or focusing.
Since a parabolic mirror focuses a laser beam by reflecting it rather than by passing and bending it, it is possible to cool the mirror from its back with high efficiency. For example in a high-output CO.sub.2 laser, it is possible to cool such a mirror with high efficiency by forming its backside from a material having a high thermal conductivity such as copper and circulating cooling water along the back of the mirror. For this reason, parabolic mirrors are frequently used in applications where high-output laser energy is needed such as laser beam welding and laser beam surface treatment.
Theoretically, a parabolic mirror can condense a laser beam to a diffraction limit with no aberration. But actually, an incident laser beam necessarily has a divergence angle, so that its optical axis (and thus its incident angle) may tilt more or less. If this happens, the quality of the condensed beam drops markedly. Thus, in order to focus a laser beam with a parabolic mirror, the optical axis of the incident laser beam has to be adjusted with a high degree of accuracy. This work is difficult with a conventional device.
Even if the optical axis is initially adjusted with a high degree of accuracy, a tilt in the optical axis can occur if any optical element in the condensing device should shift or if the operation of the laser beam generator or the laser beam transmitter becomes unstable. This leads to reduced beam-condensing properties of the condensing device. This in turn detrimentally affects the laser beam machining efficiency. If this happens, it is sometimes necessary to stop the entire device in order to readjust the optical axis.
Off-axis parabolic mirrors, which are used in many laser beam condensing optical devices, have a smooth mirror surface having small surface roughness and an accurate parabolic shape. Such a surface is formed by Single-Point Diamond Turning (hereinafter called SPDT) using a diamond cutting tool and an ultraprecise lathe.
To form a non-off-axis parabolic concave reflecting mirror such as a spherical mirror by SPDT, a mirror material is machined with a cutting tool by clamping it to the rotation center of a lathe spindle. To form an off-axis parabolic mirror, which is a part of a paraboloid of revolution, a mirror material is machined with a cutting tool by clamping it to the lathe at a point spaced a predetermined distance from the spindle axis.
Thus, it is extremely difficult to form such a parabolic mirror with high precision because the cutting tool tends to break due to unstable rotation of the spindle and/or intermittent cutting. Also, the off-axis distance is limited by the machining diameter of the ultraprecise lathe, so that it is impossible to machine a parabolic mirror having a long off-axis distance, i.e. a long focal distance.
An object of this invention is to provide a method and device for condensing a laser beam which uses non-parabolic reflecting mirrors to obviate the above problems associated with parabolic mirrors. With the method and device the beam can be sufficiently and stably condensed while maintaining a long focal distance, and there is no need for SPDT machining to provide an off-axis reflecting surface and a long focal distance.